The present invention relates generally to rotameter-type flowmeters, and more particularly to a flowmeter having improved end stops for enhancing the performance of the flowmeter.
Flowmeters of the variable-area conduit type, or "rotameters", are well-known instruments for measuring the rate of flow of fluids. They generally comprise a vertically-oriented, graduated glass conduit having a tapered longitudinal bore. A weighted float, usually of spherical, cylindrical or annular shape, is disposed in the bore for longitudinal travel along the length of the bore. To center the float within the bore and to guide its longitudinal travel therein, interior ridges extend the length of the conduit parallel to its axis.
End stops or plugs may be inserted into the ends of the bore to prevent leakage of the fluid and to retain the float within the bore. The end stops may provide channels therein to permit fluid to flow from an inflow source, through a channel within an inlet end stop, and into the bore and, subsequently, from the bore through a channel within an outlet end stop.
When no fluid is flowing through the bore, the float will generally rest at the bottom of the conduit upon the inlet end stop. As fluid flow through the bore is initiated and increased above a threshold rate, the pressure exerted on the bottom of the float by the upward flow lifts the float from the inlet end stop upward into the tapered bore.
As the float proceeds upwards, guided by the ridges, the cross-sectional area of the bore gradually increases, creating more and more clearance between the float and the inner surface of the conduit. The upward force exerted on the float by the upward fluid flow thus weakens, as more and more of the flow is diverted around the float through the gaps between the floatguiding ridges, until the upward force precisely offsets the weight of the float, bringing the float into vertical equilibrium. When the flowmeter is properly calibrated to account for the density and viscosity of the flowing fluid as well as for any other variables in the system, the equilibrium position of the float along the length of the graduated conduit can be translated into an accurate measurement of the rate of fluid flow.
Such variable-area conduits are usually made of relatively thin-walled glass and are thus somewhat fragile. The end stops, typically made of polypropylene or polytetrafluoroethylene, may be press-fitted into both ends of the conduit to protect the tube against stresses and may or may not include o-rings to effect fluid-tight sealing of the bore.
U.S. Pat. No. 3,183,713, entitled "Flowmeter", discloses a flowmeter which uses end stops in this manner. The end stops disclosed therein are made of self-lubricating plastic and have portions extending into the conduit to engage and support the float. Openings are provided in the end stops through which fluid flow may enter or exit the bore.
The design of the end stops significantly effects the operational parameters of the flowmeter. In particular, the threshold flow rate for a given fluid to lift the float is largely dependent on the relative configurations of the inlet end stop and the float. Spherical and solid cylindrical floats are typical in the prior art, but due primarily to their large inertias, such floats have relatively high threshold flow rates and may not function well or at all when using less dense or less viscous fluids.
Lighter, annular floats have been employed to increase operational range, particularly on the high end. Because annular floats permit some fluid to flow through the float without exerting upward force thereon, such floats require greater fluid flow to raise them to the same height as for a solid float. Thus, an annular float can measure higher rates of flow than a solid float before reaching the upper end stop.
Annular floats have not been as effective, however, in measuring low rates of fluid flow. Because most end stops provide for a central airflow therethrough, and an annular float initially rests directly upon the end stop, a substantial amount, if not most of the fluid flow may pass through the longitudinal hole in the annular float. At low flow rates, there is insufficient upward force to lift the float from the bottom end stop and maintain it on an elevated position.
Another performance limitation of certain flowmeter designs is a phenomenon whereby a high flow rate or a sudden increase in flow rate causes the float to impinge against the outlet end stop with sufficient force to cause "sticking" of the float to the outlet end stop. Depending upon the configurations of the outlet end stop and the float, it may be possible for the float to wedge itself into the outlet end of the conduit, obstructing the fluid flow and rendering the flowmeter inoperative until the float may be jarred or vibrated loose.
Thus, many current flowmeters have their useful operational range limited by end stop and float designs which require a high threshold flow rate to lift the float off the inlet end stop, and which are susceptible to sticking of the float in the outlet end stop under certain conditions.